To get a dynamic entity’s postion, use entity.position.getValue(viewer.clock.currentTime). There’s an example of this in this code example that gets the velocity at each timestamp in order to create a label with the vehicle’s speed: https://sandcastle.cesium.com/index.html?src=Time%20Dynamic%20Wheels.html.
When sharing your code, please follow the instructions here to share a Sandcastle example on the forum: How to share custom Sandcastle examples